1. Field of the Invention
This invention relates to a method for recovering oil from tilted oil reservoirs, preferably 10 degrees or more, having an underlying body of water in contact with the oil zone. More particularly, this invention relates to injecting a miscible solvent into the lower portion of the reservoir in the vicinity of the interface between the water and the oil wherein the solvent fingers upwardly into the oil reducing its viscosity and thereafter driving any remaining solvent and the lower viscosity solvent-oil mixture through the reservoir by water towards a spaced production well where reservoir oil is recovered.
2. Description of the Prior Art
In the recovery of oil from tilted oil-bearing reservoirs, it is known to inject a fluid which is miscible with the oil into the upper portion of the reservoir and drive the fluid down through the reservoir so as to displace the oil toward a production well in the lower portion of the reservoir where the oil is recovered. This type of process is disclosed in U.S. Pat. Nos. 3,223,157 to Lacey et al. and 3,312,278 to Warden. In this type process, the miscible fluid does not tend to finger and dissolve into the oil because of the gravity effect tending to "float" the fluid above the reservoir oil.
As disclosed in U.S. Pat. No. 3,223,157 to Lacey et al. discussed above, for economic reasons the solvent is normally injected as a slug followed by another fluid such as a gas or an aqueous fluid or a combination of water and gas to drive the solvent slug and the oil through the reservoir.
In displacement processes in general, the idea sought after is piston-like displacement. That is, the displacing fluids should ideally present a flat front to the oil in the reservoir and displace it uniformly through the reservoir. Most miscible slugs are followed by water to drive them through the reservoir. Moreover, since most miscible solvent slugs used are light hydrocarbons with viscosities and densities less than the reservoir oil, the solvent will tend to finger through the more viscous reservoir oil due to viscous fingering and gravity segregation, destroying piston-like displacement. The water will have a tendency to follow the solvent finger resulting in premature breakthrough of the displacing medium water.
The present process takes advantage of the fingering effect of a miscible solvent having a viscosity and density less than the reservoir oil by injecting a large amount of solvent in the lowest portion of a tilted reservoir. The less viscous solvent fingers into the oil reducing its viscosity and making the oil easier to produce. In the case of a tilted reservoir having an underlying body of water, the solvent is injected in the vicinity of the oil-water interface. Water is then injected into the reservoir to drive the solvent and oil toward a production well where the oil is recovered. In the present process, the rate of water injection is controlled slow enough so that the more dense water does not finger into the solvent or solvent-oil mixture destroying piston-like displacement and resulting in the water bypassing the oil and breaking through the production well.